1. Field of the Invention
The present invention relates to a sheet feeding apparatus and to an image forming apparatus employing the sheet feeding apparatus. More particularly, the present invention relates to a device configured to lock a lifting plate provided in a sheet accommodating unit of the sheet feeding apparatus.
2. Description of the Related Art
Image forming apparatuses, such as a printer and a copier, adapted to feed sheets to an image forming unit by a sheet feeding apparatus have nowadays been widely used. Some sheet feeding apparatuses are configured so that a sheet cassette is demountably mounted in an image forming apparatus body so as to facilitate, for example, accommodation and replacement of sheets. With this configuration, the sheet cassette is mounted in the image forming apparatus body. Then, sheets accommodated in the sheet cassette are fed by a pickup roller to the image forming unit.
Meanwhile, such a sheet cassette has a lifting plate that supports a sheet and that is pivotable upwardly and downwardly. When a sheet is fed to the image forming unit of some image forming apparatus, the lifting plate is pushed by a spring toward the pickup roller to press the sheet against the pickup roller. Thus, the sheet feeding apparatus obtains a feeding pressure required to feed the sheet.
In a case where sheets are replenished in the sheet cassette in some image forming apparatus, or where sheets are replaced with those stored in the sheet cassette therein, the sheet cassette is drawn out of the image forming apparatus body. Then, the lifting plate is pushed down against a pushing force of the spring. The lifting plate is locked by a locking mechanism at a descent position to which the lifting plate is pushed down. With this configuration, the replenishment and the replacement of sheets of the sheet cassette can easily be performed.
The locking of the lifting plate by the locking mechanism is released when the sheet cassette is mounted in the apparatus body. Consequently, the lifting plate rises toward the pickup roller when the sheet cassette is inserted into the image forming apparatus body. Thus, the feeding of sheets is enabled.
However, the position of the locking mechanism is such that users can easily operate the locking mechanism. Thus, when replenishing or replacing the sheets, a user may release the locking of the lifting plate by the locking mechanism so that one end of the lifting plate jumps up. Moreover, to fix the lifting plate to the descent position (or lock position), it is necessary to once push down the lifting plate to the descent position before sheets are stacked. This is troublesome.
To solve this problem, a technique discussed in the U.S. Pat. No. 5,564,690 has been developed. This technique is described below with reference to FIG. 6. As shown in FIG. 6, a locking mechanism 50 adapted to lock a lifting plate 102 at a descent position in a sheet cassette 100 is provided substantially at the center of a cassette body 101 of the sheet cassette 100 to be at a lower position than the lifting plate 102. Thus, the locking mechanism 50 can be prevented from erroneous operation by users.
To release the locking of a member by the locking mechanism 50 in a case where the locking mechanism 50 is provided at such a position, a lock release member 60 is provided on a bottom plate 70 in a lower part of an image forming apparatus body 200. FIG. 7 illustrates the locking mechanism 50 in detail. The locking mechanism 50 is pivotably provided in the sheet cassette 100, and is fixed in a position in which the locking mechanism 50 pushes down the lifting plate 102 by being caught onto a latch part 102a of the lifting plate 102. As viewed in FIG. 6, the sheet cassette 100 is moved from right to left and is mounted to a cassette mounting unit 201.
At that time, a releasing lever 51 provided in the locking mechanism 50 abuts against the lock release member 60 provided on the bottom plate 70 in the lower part of the apparatus body 200. Then, the locking mechanism 50 is turned in the direction of an arrow shown in FIG. 7. Consequently, the locking of the lifting plate 102 is released. Accordingly, the lifting plate 102 is lifted and pushed by a pushing force of a spring 103 toward a pickup roller 203 provided in the apparatus body 200. Thus, the lifting plate 102 is moved to a sheet feeding position to enable the feeding of sheets.
Meanwhile, a guide unit 65 having a slope 66 is provided on an entrance side surface of the cassette mounting unit 201 of the image forming apparatus body 200. A sliding-contact unit (not shown) protruding laterally from the lifting plate 102 in the sheet cassette 100 slide-contacts on the slope 66. Thus, the lifting plate 102 is pushed down and up along the slope 66 when the sheet cassette 100 is mounted in and is demounted from the image forming apparatus body 200.
When the sheet cassette 100 is drawn out of the image forming apparatus body 200, the lifting plate 102 descends along the slope 66 of the guide member 65 shown in FIG. 6 against the pushing force of the spring 103. The lifting plate 102 is fixed by the locking mechanism 50 at a position, at which the lifting plate 102 is pushed down, before the sheet cassette 100 is drawn out. Thus, the lifting plate 102 is locked in conjunction with an operation of drawing the sheet cassette 100 out of the apparatus body 200. This eliminates necessity for pushing down the lifting plate 102 to stack sheets. Consequently, operability can be enhanced.
Meanwhile, the locking mechanism 50 can be moved to a lock position, at which the lifting plate 102 is locked, and a lock release position at which the locking of the lifting plate is released. When moving to the lock position, the locking mechanism 50 abuts against an abutment part 55 of a sheet cassette body 101, as illustrated in FIG. 7, so that the locking mechanism 50 cannot move in a direction opposite to the lock release position. With this configuration, when the locking mechanism 50 moves to the lock position, the pushing force of the spring 103 locks the lifting plate 102. Consequently, the lifting plate 102 can surely be locked by the locking mechanism 50.
Recently, along with diversification of sheets used, sheet cassettes have been provided in an image forming apparatus as multiple stages. In such an image forming apparatus, a lower-stage sheet cassette (not shown) is mounted under, for example, the sheet cassette 100 shown in FIG. 6. A lower-stage pickup roller (not shown) feeds sheets. A cassette conveyance passage 101a, through which a sheet is conveyed from a lower-stage sheet cassette, is provided in each of the sheet cassettes 100. Also, an opening 70a, through which a sheet is supplied from an immediately lower stage sheet cassette to the cassette conveyance passage 101a of the upper stage sheet cassette 100, is provided in the bottom plate 70 (a partition plate that partitions an accommodating unit, which accommodates the upper stage sheet cassette, from an accommodating unit which accommodates the immediately lower stage sheet cassette) of the image forming apparatus body 200.
In this configuration, it is necessary that the lock release member 60 is provided at one of anterior and posterior positions of the opening 70a in a mounting direction of the sheet cassette 100, i.e., at a position (hereunder referred to as an interior position), where the sheet cassette 100 abuts against the locking mechanism 50 just before the mounting of the sheet cassette 100 is completed, or at an entrance side position of the cassette mounting unit 201.
In a case where the lock release member 60 is provided at the interior position of the cassette mounting unit 201, and where the locking of the lifting plate 102 is released when the sheet cassette 100 is mounted in the apparatus body 200, the lifting plate 102 is rapidly pushed up by the spring 103. In such a case, an upward inertia force acts upon each sheet. Thus, when a sheet abuts against the pickup roller 203, a large force is applied to the sheet. Consequently, the posture of a sheet is disturbed when stacked.
In such a case, the sheet feeding performance of the apparatus is degraded. This results in occurrences of sheet feeding failures, such as skew-feeding and non-feeding of sheets. Thus, sometimes, it is necessary to restack the sheets. Also, when a sheet rapidly bumps up against the pickup roller 203, collision noises may be generated, or breakages of the sheet cassette 100 and the pickup roller 203 may be caused.
Meanwhile, in a case where the lock release member 60 is provided at the entrance side of the cassette mounting unit 201, as indicated by dashed lines in FIG. 6, the locking of the lifting plate 102 can be cancelled in an early stage of insertion of the sheet cassette 100. In this case, the lifting plate 102 can moderately be lifted along the slope 66 of each of the guide members 65 provided at both ends of the cassette mounting unit 201. Consequently, each sheet can be prevented from rapidly bumping up against the pickup roller 203.
However, in this case, the lock mechanism 50 cannot turn in a direction opposite to a direction indicated by an arrow shown in FIG. 7. Thus, when the sheet cassette 100 is drawn out, the releasing lever 51 of the locking mechanism 50 is caught in the lock release member 60. Thus, the sheet cassette 100 cannot be drawn out. Accordingly, in the conventional apparatus configured so that the cassette conveyance passage 101a is provided in the sheet cassette 100 and that a sheet is fed from a lower stage sheet cassette through the cassette conveyance passage 101a to the image forming unit, the locking mechanism 50 adapted to lock the lifting plate 102 cannot be provided under the sheet cassette 100.